Range Rover P38
19
FUEL SYSTEM
NEW RANGE ROVER
54
DESCRIPTION AND OPERATION
The heated oxygen sensors age with mileage, which
will cause an increase in their response time for
switching from rich to lean and lean to rich. The
increase in response time influences the closed loop
control and leads to progressively increased
emissions. If the response rate is diagnosed to be
exceeding a preset threshold, an error code will be
stored in the ECM and the MIL warning lamp will be
illuminated (NAS only).
The heated oxygen sensor is protected by an outer
tube with a restricted flow opening to prevent the
sensor’s ceramics from being cooled by low
temperature exhaust gases at start up. The
pre-catalytic sensors are identified by three slots in
the protective tube, whereas the post-catalytic
sensors have four square indentations and a hole in
the end of the protective tube (NAS only).
NOTE: The maximum working temperature
of the tip of the HO
2
S sensor is 930
°
C
(1706
°
F); temperatures higher than this
will damage the sensor.
The heater elements are controlled by a PWM signal
from the ECM. The heater elements are operated
immediately following engine start and also during low
load conditions when the temperature of the exhaust
gases is insufficient to maintain the required sensor
temperatures. The heater element warms the sensor’s
ceramic layer from the inside so that the sensor is hot
enough for operation. After start up, the sensors are
ready for closed loop control within about 20 to 30
seconds.
If the heater element fails, the ECM will not allow
closed loop fuelling to be implemented until the sensor
has achieved the required temperature. A diagnostic
routine is utilised to measure both sensor heater
current and the heater supply voltage, so its
resistance can be calculated. The function is active
once per drive cycle, as long as the heater has been
switched on for a pre-defined period and the current
has stabilised. The PWM duty cycle is carefully
controlled to prevent thermal shock to cold sensors.
The pre-catalytic and post-catalytic converters are not
interchangeable, and although it is possible to mount
them in transposed positions, their harness
connections are of different gender and colour:
•
Upstream sensors have orange connectors.
•
Downstream sensors have grey connectors.
It is important not to confuse the sensor signal pins;
the signal pins are gold plated, whilst the heater
supply pins are tinned, mixing them up will cause
contamination and effect system performance with
time.
NOTE: Sensor voltage is most easily
monitored using "Testbook".
If a heated oxygen sensor should fail, the following
symptoms may be observed:
•
Default to open loop fuelling on the catalyst bank
with the failed sensor.
•
If sensor get crossed, the engine will run
normally after the initial start, but then become
progressively unstable. One bank will clamp at
the maximum rich level, and the other bank will
clamp at maximum lean. The system will then
revert to open loop fuelling.
•
High CO reading
•
Excess emissions
•
Strong smell of hydrogen sulphide (H
2
S) until the
ECM defaults to open loop fuelling
•
MIL lamp illuminated (NAS only)
A heated oxygen sensor failure is likely to occur for
the following reasons:
•
Damaged or incorrectly fitted sensor
•
Sensor open circuit or disconnected
•
Short circuit to vehicle supply or ground
•
Stoichiometric ratio outside the correct operating
band
•
Contamination from leaded fuel or other sources
•
Change in sensor characteristics - Chemical
Shift Down (CSD)
•
Sensors from LH and RH banks crossed
•
Air leak into exhaust system (cracked pipe / weld
or loose fixings)